Data storage devices such as hard disk drives, tape drives, and the like typically utilize magnetic storage media from which data is read and/or to which data is written. A magnetic storage medium typically includes a layer of magnetizable material that is capable of being selectively magnetized to produce magnetic transitions that represent the stored information. Technology improvements continually reduce the area on a storage medium required to represent individual bits of information, and thus permit greater amounts of information to be stored on the storage medium. A hard disk drive, for example, may be constructed using one or more magnetic disks, with each disk being segmented into a number of concentric tracks, and with each track being segmented into polar arrays of sectors. Increases in storage density have decreased both the track pitch and the sector size for the magnetic disks utilized in modern hard disks drives, resulting in increasingly greater storage capacities.
As storage densities have increased, however, the sensitivity of data storage devices to media defects have also increased. In particular, for magnetic disks and other magnetic storage media, imperfections in the magnetic layers thereof can render certain areas of a storage medium unacceptable for use in storing information. Such media defects can range in size from very small, e.g., affecting only a few sectors occupying a few tracks, to relatively large, e.g., affecting many sectors occupying a large number of tracks. Furthermore, as storage densities increase, a media defect of a given size will tend to adversely affect a proportionately larger numbers of sectors and tracks.
Despite adherence to proper manufacturing techniques, at least some media defects occur in practically every manufactured disk. Conventionally, these media defects are accounted for in the electronics of the disk drive by effectively removing “defective” sectors from the storage areas recognized as being usable by the drive. In many disk drive designs, Surface Analysis Testing (SAT) is performed during manufacture of a disk drive, e.g., in connection with the initial low-level formatting of the disk drive. During SAT, defective sectors on a disk are identified, and then stored in a table and remapped to usable substitute, or “spare”, sectors disposed in non-defective areas of the disk. Subsequently, whenever a drive attempts to access a defective sector, the spare sector to which it has been mapped can be accessed in its place.
Similarly, during the normal operation of some disk drive designs, additional defects may be detected, e.g., due to the detection of soft errors during unsuccessful read operations. In such instances, a process similar to the remapping that occurs during SAT may be used to remove newly-detected defects from the storage areas recognized as being usable by the drive.
It has been found, however, that due to various factors such as thermal and mechanical stresses and other media damaging mechanisms, some media defects have a tendency to increase in size over time. As such, media defects can sometimes affect sectors that are adjacent to previously-identified defective sectors, but that were originally identified as being non-defective during SAT. A result of the growth of a media defect into an area occupied by a sector identified as being non-defective can be the loss of information stored in the sector.
Conventionally, to accommodate for media defect growth, various sectors located adjacent identified defective sectors, which are commonly referred to as “pad” sectors, may be marked as unusable during SAT and/or during later operational remapping of new defects, thus effectively creating a buffer around each media defect. From the standpoint of a drive's electronics, pad sectors are treated the same as defective sectors, and as such, the buffer areas circumscribing each media defect are excluded from being used to store information.
The use of pad sectors, while providing the ability to accommodate for media defect growth, creates an undesirable tradeoff between storage capacity and reliability. Increasing the area of the buffers around each media defect increases reliability as a comparatively greater amount of media defect growth can be accommodated for, but at the expense of decreasing the amount of usable area on a disk. Likewise, selection of a minimal amount of pad sectors around each media defect maximizes storage capacity, but renders the disk drive more susceptible to data loss as a result of media defects expanding beyond their assigned buffer areas.
Therefore, a significant need has arisen in the art for an improved manner of handling the growth of media defects on magnetic disks and other forms of data storage media, and in particular, for an improved manner that more appropriately balances storage capacity and reliability.